SP-259CD The American Concrete Institute (ACI) Committee 231, Properties of Concrete at Early Ages, has sponsored a full-day technical session on the Transition from Fluid to Solid: Re-examining the Behavior of Concrete at Early Ages at the ACI Spring Convention, San Antonio, Texas, March 15-19, 2009. This special publication contains the twelve papers presented at this session. The subject matter of these papers includes: (1) the development of concrete properties and microstructure at early ages, (2) test methods for assessing early-age volume change and cracking potential, (3) construction operations timing, (4) computer simulations of early-age behavior, and (5) mechanisms that end the concrete dormant period.

Ultrasonic wave reflection (UWR) has been used to monitor hydration and strength development of concrete. UWR measures the changes in reflected ultrasonic waves at the interface between a buffer material and hydrating cement paste. To monitor the subtle changes during early hydration it is necessary to use a buffer with low acoustic impedance, close to that of cement paste. In this research, UWR measurements on hydrating Type I portland cement are performed using a high impact polystyrene (HIPS) buffer. Both S-waves and P-waves are analyzed simultaneously to develop and extend the use of UWR to monitor early stiffening of cement paste. The penetration resistance test (ASTM C 403) and temperature rise of cement paste are used to correlate stiffening characteristics. The UWR responses show good correlation with results from temperature rise and penetration resistance. The onset of stiffening is the same for penetration resistance and both P- and S-wave UWR, and nearly the same for temperature rise. It is found that the HIPS buffer can provide sensitive measurement on the early age stiffening of cement paste.

Structural build-up that occurs during the induction period is of particular interest to users of self-consolidating concrete (SCC) since it can affect the workability of concrete. A novel experimental device based on scanning laser microscopy was used to directly monitor particle flocculation in SCC cement pastes. This is one of the few studies in which this experimental method has been used to study flocculation in concentrated suspensions. This paper discusses the results from a study that was carried out to investigate the flocculation and floc properties in SCC cement pastes. Results show that the floc network is immediately broken down by superplasticizers and that the rate of reflocculation decreases when the water-to-cement (w/c) ratio is decreased. An increase in w/c ratio resulted in a reduction in floc strength. Results show that viscosity modifying agents can induce flocculation due to different flocculation mechanisms.

A simple but challenging experiment was carried out to measure concrete temperature, air content, unit weight, slump, setting (penetration resistance), heat release, maturity, and compression strength over a 28-day period beginning with discharge from the chute of a concrete truck. It was thus demonstrated that concrete’s transition from liquid to solid is represented continuously by maturity and by heat release, but it is more commonly recorded in terms of three phases in concrete development: slump loss, setting, and strength gain. The paper describes how these phases overlap each other and are related to concrete temperature, heat release, and maturity.

The evolution of early age mechanical properties and volume change of cement paste is performed through Finite Element analysis on a 3D computer-generated cement paste. The time evolution of the hydrating microstructure is generated by µic(mike), a vectorial hydration model which takes into account the Particle Size Distribution (PSD) of anhydrous cement particles, the w/c ratio, the filler content and different hydration kinetics mechanisms such as nucleation, growth and diffusion. The microstructure geometry is then discretized into a finite element mesh. At each hydration step, the capillary depression is computed according to Laplace-Kelvin equation and applied on the pore space generated by the hydration model. Then, the autogenous shrinkage corresponds to the overall load-free deformation of the computational volume. Two constitutive models are used. The first one is a purely elastic model where macroscopic stress depends on the total porosity only. The second one is a poroelastic model which takes into account the fluid-solid interaction and the de-saturation effect. In parallel to the modeling work, a systematic experimental study has been performed on series of white cement pastes prepared different finenesses and various water-cement ratios. Many characterization techniques were used in the experimental study: chemical shrinkage, evolution of relative humidity, mercury intrusion porosimetry (MIP), x-ray diffraction (XRD), linear and volumetric autogenous shrinkage and ultrasonic wave propagation measurements. The numerical results are compared with experiment data and it is shown that the poroelastic model provides the best agreement to the experimental results. The remaining gap between the modeling and the experiment is discussed and future developments are outlined.